There is a three-way or four-way valve used for piping of air conditioner and the like as prior art of the present invention (refer to the patent reference 1 given below). Inside the piping of the refrigeration cycle in the air conditioner an operational fluid such as CO.sub.2 or the like is used as a refrigerant. In the refrigerant cycle using CO.sub.2 as operating fluid, generally the service pressure range becomes more than ten times compared with those of conventional refrigerants. The fact that the service pressure is high causes various problems to a switching valve. FIG. 4 illustrates a refrigeration cycle of this air conditioner. Also FIG. 5 illustrates a heating cycle of the air conditioner. In addition, FIG. 6 shows a full cross-sectional view of a three-way valve used in the cycles of FIG. 4 and FIG. 5.
Construction of the solenoid valve 130 will briefly be described by using FIG. 6. In the valve main body 104, there is disposed a solenoid portion, not shown, to the right hand side of the figure where the reference numeral 130 is placed, in which the solenoid portion is integral with the valve main body 104. And the spool 106 is actuated by supplying an electric current to the solenoid portion. In the valve main body, there is disposed a slide surface 104A on the circumference of an axially extending bore which guides the spool 106. Inside the non cross-sectional portion which is located in the right hand side of FIG. 6, the spool 106 and a spool valve being integral with the spool 106 are disposed in which this spool valve opens or closes the fluid passage at fluid inlet port 112 under discharge pressure and fluid outlet port 113 under control chamber pressure.
Also the fluid inlet port 112 under discharge pressure and the fluid inlet/outlet port 111 under suction pressure are communicated by means of piping having an orifice therebetween. O-ring 125 is mounted in an annular grove which is formed by a recess portion disposed in the valve main body 104 and a support plate 126 in order to prevent the fluid under suction pressure form leaking through the gap between the spool 106 and the sliding surface 104A toward atmospheric chamber 107. There is disposed a spring 109A in the atmospheric chamber 107 which is retained by a spring retainer 109. This spring 109A resiliently urges the spool 106 to the solenoid portion side.
In the solenoid valve 130 thus constructed, O-ring 125 forms an intermediate fit with the spool 106 and the valve main body 104 in order to prevent the fluid at suction pressure in the fluid inlet/outlet port 111 from leaking through the clearance gap between the spool 106 and the sliding surface 104A. This causes the fluid at suction pressure to act on the O-ring 125 from one direction such that the O-ring 125 undergoes elastic deformation in a radial direction. When the solenoid portion is energized by the current supplied to the solenoid portion and actuates the spool 106, frictional resistance of the spool 106 increases due to friction thereof with the O-ring 125. More particularly, since fluid under a highly pressurized suction pressure needs to be sealed in air compressors and the like as previously described, if this fluid of suction pressure acts on the O-ring 125 through between the spool 104 and the sliding surface 104A, then the O-ring 125 undergoes elastic deformation such that the O-ring 125 is squeezed and increases sliding friction relative to the spool 106. As the result, the intensity of the current supplied to the solenoid portion does not remain proportional to the operational speed of the spool 106 and the response of the solenoid valve 130 for controlling the fluid flow may be deteriorated. And besides, the seal ability decreases as the inner circumferential seal face of the O-ring 125 is subject to wear due to press-contact thereat.    Patent reference 1: Japanese Patent Laid-Open Publication No. 2000-193124 (FIG. 3 and FIG. 4)